1. Field of the Invention
This invention relates to an athermalized optical head which is adapted to support a light source and an optical element, and more particularly, to such an optical head which is particularly suitable for use with a laser diode.
2. Description of the Prior Art
Laser diodes are used in certain types of scanning apparatus to project a laser beam onto a polygon mirror which is adapted to scan the beam onto a recording medium. One of the problems in such apparatus is in maintaining the focus of a laser spot, since the distance between the laser diode and a collimating lens changes with changes in ambient temperature. As the ambient temperature changes, even slightly, the materials in the supports for the laser and the collimator expand or contract. This movement changes the relative positions of the laser and the collimator and causes the laser spot to be out of focus. The problem of maintaining a laser diode an exact distance from a lens was recognized in U.S. Pat. No. 4,295,152. In this patent, an optical coupler is provided with a telescoping housing which can be adjusted to position a semiconductor laser an exact distance from a lens. There is no solution in this patent, however, to the problem of maintaining a constant distance between the diode and the lens with changes in ambient temperature.
It is common in apparatus using laser diodes to control the temperature of the laser diode in order to maintain a constant output from the laser diode. In U.S. Pat. No. 4,403,235, for example, there is disclosed an imaging system which comprises a laser diode which is surrounded by a thermoelectric cooling element. Such a device is effective in maintaining the temperature of the laser diode within an acceptable range. However, as the device pumps heat away from the laser diode, it causes localized thermal gradients which add distortion to expansion and contractions already present in the laser and collimator mounts. Thus, the thermoelectric cooling element contributes to the problems of maintaining a constant distance between the laser diode and the collimating lens.